How did pre-flood people understand the constellations and planets?
Pre-flood people understood the constellations and planets as a written form of the Word of God. They were expected to interpret God's message from the stars, and the pre-flood patriarchs had a good understanding of that Word simply by studying them. Satan later adulterated this original revelation.
Les explains that Scripture indicates the Word of God was written in the stars once upon a time. Adam was able to comprehend it, and the pre-flood people comprehended that the very makeup of the constellations and planets was to be interpreted as the Word of God. Satan took it and adulterated it, but in its original format, the pre-flood patriarchs had a pretty good understanding of the Word of God just by studying the stars. This fits with Paul's teaching in Acts 17 that even pagan Greeks, without written scripture, should have had a knowledge of the Creator God behind it all.